‘Shell element’ means a structural part that is relatively thin with regard to its other dimensions, extended along two curvilinear directions whose shape can vary from that of a flat panel to an ellipsoid portion, or more complex shapes with variable double curvature. For example, it can include elements of aircraft wings or fuselages, ship hulls or tanks.
According to the prior state of the art, FIG. 1, the assembly of two shell elements (11, 12) placed side by side is realized with an intermediate part such as a collar (13) or a splice plate. Said collar extends perpendicular to and on both sides of the assembly plane (10). The two shell elements bear on the collar perpendicular to the assembly plane and are fastened to it by means of fasteners (21, 22) extending perpendicular to the contact surfaces between the collar and said shell elements. If the assembly is subjected to traction or compression stresses whose direction is perpendicular to the assembly plane, the force flow corresponding to this stress flows from one element to the other passing by the collar. Thus, this force flow flows from a first shell element (11) to the fasteners (21), subjecting them to shear and burring stresses, then from the fasteners (21) to the collar (13) and from the collar to the second shell element (12), passing again via the fasteners (22). So that this force flow can flow with no risk of damaging the interface structures, especially if the stresses are cyclical, the contact surfaces (101, 102) between the collar and the shell elements must be perfectly adjusted in both shape and perimeter, and the fasteners must be mounted with no play in their housings. These conditions are difficult to comply with in the case of large-sized parts. In particular they entail very tight shape tolerances for the parts present and the counterbored mounting of the fasteners. That is to say that the parts are pre-assembled and that the drilling of holes intended to receive the fasteners is realized simultaneously through the pre-assembled parts held in place. This operating procedure, essential for the resilience of the assembly, is however very penalizing in terms of time. Most often, it requires having access to each side of the shell elements to perform the drilling and fit the fasteners.
In the case in which the elements to be assembled are made of composite materials, such as a laminate of layers of continuous fibers in a resin, the thickness of the elements must be greatly increased in the neighborhood of the assembly plane. In effect, this type of material has only poor resistance to burring. Consequently, when the force flow passing through the assembly is high, it requires the use of fasteners that have a larger diameter, and are therefore more spaced out and cover a greater length, as well as the installation of a greater number of fasteners compared to the assembly of two metal shell elements. This local reinforcement of the shell elements makes them considerably more rigid so that they are even more difficult to adapt in shape and perimeter. In addition, the assembly of elements thus reinforced deviates the force flow from the neutral axis of the shell elements, so that the combination of this force with the end rigidity of the assembled elements gives rise to parasitic bending stresses in the structure resulting from the assembly.
The same argument can be made with regard to the shearing flows, i.e. stresses that act parallel to the assembly plane.
A device is known from the prior state of the art, FIG. 2, in particular from European patent EP1234984 and application US2003205011, both in the name of the applicant, for assembling a composite shell element (13) and a metal shell element (14) assembled by surfaces parallel to the assembly plane by means of traction shafts (23) arranged perpendicular to this assembly plane. Such an assembly allows two shell elements to be connected without utilizing a collar. However this device cannot be transposed to the case of the assembly of two composite shell elements. In effect, in this case the traction shafts must be perfectly adjusted in their housings and over a great length, so as to transmit the shearing flows from one element to the other without risk of damaging their housing by burring. Such a shaft is considered to be adjusted over a great length when its centering length in the part is greater than its diameter, generally 2 times its diameter. In the case of this prior state of the art, this great installation length of the shaft is only present on the composite side. On the metallic side, the burring resistance of the material forming the shell element is much greater, so that the shaft is only centered over a smaller distance, less than or equal to its diameter. In this way the shafts can be pre-positioned on the side of the element made of a composite material before the two elements are butted up, then introduced into the metal element (14) and pulled through it so as to bring the two shell elements closer together, until their assembly surfaces (113, 114) are in contact.
The shaft's short centering length in the metallic portion makes this mounting not very sensitive to slight alignment defects between the holes of two opposite shell elements.
If greater alignment precision is required, the shape of the interface on the metallic side, with its turned out edge (141), allows the pre-assembled elements to be counterbored via the metallic side. However, in the case of a shell element made of a composite material, the turned out edge of the prior state of the art cannot be reproduced without a decline in the mechanical properties of this extremity, which would then work in expansion with respect to the stresses perpendicular to the assembly plane. This mode of stress may be adverse for composite materials. Even if such a shape were realized in composite, the design constraints on the latter's mechanical resilience would deviate the traction shafts from the shell element's neutral axis and would give rise to parasitic moments. If the force flows to be passed through the interface are such that the conditions for burring resistance require a significant contact length between the traction shaft and the hole, it is necessary to have an adjustment tolerance between the diameter of the shaft and the diameter of the hole that can absorb the slight alignment defects between opposite holes of the two shell elements. However this type of mounting, the ability to transmit high cyclical forces, must be realized without play and therefore this type of mounting does not have this adjustment tolerance able to absorb said alignment defects. Given the impossibility of counterboring, the mounting cannot therefore be realized. In addition, other objects, desirable features and characteristics will become apparent from the subsequent summary and detailed description, and the appended claims, taken in conjunction with the accompanying drawings and this background.